1. Field of the Invention
The present invention relates to a semiconductor chip mounting method and apparatus for mounting a semiconductor chip on a circuit board using a flip chip bonding technique, and more particularly to a method and apparatus for preventing the circuit board from warping when heated in a solder reflow process.
2. Description of the Prior Art
In recent years, there has been a growing trend that semiconductor devices have an increased number of connection terminals according as the semiconductor devices are made finer. The wire bonding method conventionally used for terminal connection in the semiconductor chip mounting process cannot cope with such an increased number of connection terminals satisfactorily, and the flip chip bonding method, a wireless bonding method, is gaining popularity instead as a new terminal connection method.
In the flip chip bonding method, bump electrodes (solder bumps) are formed at external connection electrode portions of a semiconductor chip (flip chip) and subsequently the bump electrodes are bonded onto a circuit board by a face-down bonding technique.
FIG. 10 shows a typical semiconductor chip mounting process using the flip chip bonding.
First, flux 7 is applied onto a circuit board 2 in a manner as shown in FIG. 10 (a). Then a semiconductor chip 1 having a plurality of solder bumps 3 on its face is placed on the circuit board with the solder bumps 3 being opposed to bonding pads 6 formed on the circuit board 2 and the semiconductor chip is temporarily bonded to the circuit board 2 by virtue of the adhesive property of the flux 7 as shown in FIG. 10 (b). Thereafter, the circuit board with the semiconductor chip is put in a reflow furnace having a peak temperature of 200.degree. C. to 250.degree. C. such that the solder bumps 3 melt and then freeze in contact with the bonding pads 6, as shown in FIG. 10 (c). Finally, the flux residue is removed by the use of a solvent. In this way the semiconductor chip mounting is completed.
Materials of the circuit board to be subjected to the flip chip bonding process may include ceramic, glass, a printed wiring board, and a film board. In order to apply the technique to a variety of general-use apparatuses, use of the printed wiring board is most desirable in terms of cost reduction.
However, when the flip chip bonding is carried out to the printed wiring board, there is a problem that the printed wiring board itself is expanded or deformed by the heat of the reflow furnace in the course of solder reflow process of the solder bumps, due to which the printed wiring board tends to warp.
The warp of the printed wiring board occurs at a higher rate when the printed wiring board has a smaller thickness, and occurs more frequently particularly when the printed wiring board has a thickness of not greater than 0.5 mm.
When the warp of the printed wiring board occurs, the size of the gap between the semiconductor chip 1 and the printed wiring board 2 varies from position to position as shown in FIG. 11. As a result, there is disadvantageously produced a portion where the solder bump 3 does not reach the bonding pad 6 of the printed wiring board 2 to remain unconnected.